Generally, permanent-magnet-type rotating electrical machines are classified into two types. One is a surface-permanent-magnet-type rotating electrical machine having permanent magnets adhered to an outer circumferential face of a rotor core and an internal-permanent-magnet-type rotating electrical machine having permanent magnets embedded in a rotor core. For a variable-speed drive motor, the latter internal-permanent-magnet-type rotating electrical machine is appropriate.
The latter internal-permanent-magnet-type rotating electrical machine is described in “Design and Control of Internal Magnet Synchronous Motor,” Takeda Yoji, et al., a document of Ohm-sha Publishing (Non-patent Document 1) and Japanese Unexamined Patent Application Publication No. H07-336919 (Patent Document 1). With reference to FIG. 9, a configuration of such a conventional internal-permanent-magnet motor will be explained. Inside a rotor core 2 of a rotor 1 and close to an outer circumference thereof, rectangular hollows are arranged in a point symmetry of 360°/N where N is the number of poles. The number of the rectangular hollows is equal to the number N of poles. In FIG. 9, the rotor 1 has four poles, and therefore, four hollows are arranged in a point symmetry of 90° (=360°/4) and permanent magnets 4 are inserted into the hollows, respectively. Each permanent magnet 4 is magnetized in a radial direction of the rotor 1, i.e., in a direction orthogonal to a side (long side in FIG. 9) of the rectangular section of the permanent magnet 4 that faces an air gap. The permanent magnet 4 is usually an NdFeB permanent magnet having a high coercive force so that it is not demagnetized with a load current. The rotor core 2 is formed by laminating electromagnetic sheets through which the hollows are punched. The rotor 1 is incorporated in a stator 20. The stator 20 has an armature coil 21 that is installed in a slot formed on an inner side of a stator iron core 22. An inner circumferential face of the stator 20 and an outer circumferential face of the rotor 1 face each other with the air gap 23 interposing between them.
An example of a high-output rotating electrical machine having an excellent variable-speed characteristic is a permanent-magnet-type reluctance rotating electrical machine described in Japanese Unexamined Patent Application Publication No. H11-27913 (Patent Document 2) and Japanese Unexamined Patent Application Publication No. H11-136912 (Patent Document 3). This kind of permanent-magnet-type rotating electrical machine has a structural characteristic that a permanent magnet thereof always generates constant linkage flux to increase a voltage induced by the permanent magnet in proportion to a rotation speed. Accordingly, when a variable-speed operation is carried out from low speed to high speed, the voltage (counter electromotive voltage) induced by the permanent magnet becomes very high at high rotation speed. The voltage induced by the permanent magnet is applied to electronic parts of an inverter, and if the applied voltage exceeds a withstand voltage of the electronic parts, the parts will cause insulation breakage. It is necessary, therefore, to design the machine so that a flux amount of the permanent magnet is reduced below the withstand voltage. Such a design, however, lowers the output and efficiency of the permanent-magnet-type rotating electrical machine in a low-speed zone.
If the variable-speed operation is carried out in such a way as to provide nearly a constant output from low speed to high speed, the voltage of the rotating electrical machine will reach an upper limit of a power source voltage in a high rotation speed zone. This is because the linkage flux of the permanent magnet is constant. In the high rotation speed zone, therefore, a current necessary for providing the constant output will not pass. This greatly drops the output in the high rotation speed zone and the variable-speed operation will not be carried out in a wide range up to high rotation speed.
Recent techniques of expanding a variable-speed range employ flux-weakening control such as one described in the Non-patent Document 1. In the case of the permanent-magnet-type rotating electrical machine, a total linkage flux amount is the sum of flux by a d-axis current and flux by a permanent magnet. The flux-weakening control is based on this and generates flux with a negative d-axis current to reduce the total linkage flux amount of an armature coil. The flux-weakening control makes the permanent magnet 4 of high coercive force operate in a reversible range on a magnetic characteristic curve (B-H characteristic curve). For this, the permanent magnet is an NdFeB magnet having a high coercive force so that it may not irreversibly demagnetized with a demagnetizing field produced by the flux-weakening control.
In the flux-weakening control, flux produced by a negative d-axis current reduces linkage flux and a reduced portion of the linkage flux produces a voltage margin for an upper voltage limit. This makes it possible to increase a current for a torque component, thereby increasing an output in a high-speed zone. In addition, the voltage margin makes it possible to increase a rotation speed, thereby expanding a variable-speed operating range.
In the flux-weakening control, however, since a negative d-axis current that contributes nothing to an output is continuously passed, an iron loss increases to deteriorate efficiency. In addition, a demagnetizing field produced by the negative d-axis current generates harmonic flux that causes a voltage increase. Such a voltage increase limits a voltage reduction achieved by the flux-weakening control. These factors make it difficult for the flux-weakening control to conduct a variable-speed operation for the internal-permanent-magnet-type rotating electrical machine at speeds over three times a base speed. In addition, the harmonic flux increases an iron loss to drastically reduce efficiency in middle- and high-speed zones. Further, the harmonic flux generates an electromagnetic force that produces vibration.
When the internal-permanent-magnet motor is employed for a drive motor of a hybrid car, the motor rotates together with an engine when only the engine is used to drive the hybrid car. In this case, a voltage induced by the permanent magnets of the motor increases at middle or high rotation speed. To suppress an increase in the induced voltage below a power source voltage, the flux-weakening control continuously passes a negative d-axis current. The motor in this state, therefore, produces only a loss to deteriorate an overall operating efficiency.
When the internal-permanent-magnet motor is employed for a drive motor of an electric train, the electric train sometimes carries out a coasting operation. Then, like the above-mentioned example, the flux-weakening control is carried out to continuously pass a negative d-axis current to suppress a voltage induced by the permanent magnets below a power source voltage. The motor in this state only produces a loss to deteriorate an overall operating efficiency.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H07-336919    Patent Document 2: Japanese Unexamined Patent Application Publication No. H11-27913    Patent Document 3: Japanese Unexamined Patent Application Publication No. H11-136912    Non-patent Document 1: “Design and Control of Internal Magnet Synchronous Motor,” Takeda Yoji, et al., Ohm-sha Publishing